Nelson Mandela's only public rally in Washington is expected to be a sell-out, with about three-quarters of the 19,000 tickets already purchased for the South African nationalist's Tuesday evening appearance at the Washington Convention Center, organizers said yesterday.

Sales have been so brisk since the $10 and $25 tickets were distributed on Saturday that the rally might be sold out before Mandela's scheduled arrival in Washington Sunday, according to Sandra Hill, one of the coordinators with the Nelson Mandela District Welcoming Committee.

About half of the tickets were distributed for sale to more than 25 representatives of "sectors" that include churches, students, anti-apartheid activists, elderly people and labor, Hill said. The remaining tickets have been on sale between 5 and 7 p.m. in the lobby of the Frank R. Reeves Municipal Building at 14th and U streets in Northwest .

"It was decided that as broad a base as possible should see Nelson Mandela," Hill said. "We're trying to be very careful that all sectors are represented."

But the unadvertised ticket-divvying process has irked some Washington residents who have been scrambling without luck to find a ticket.

"Right now, I have no ticket . . . I'm willing to pay. I guess it's first-come, first-serve and I guess I'm last come," said Pat Wheeler, director of the communications office for the D.C. Department of Corrections. She finally bought a ticket yesterday after days of not knowing that they were being sold at the Municipal Building.

In the first hour of sales in the Reeves Center lobby yesterday evening, 200 people bought tickets. The tickets can be purchased only by check or money order.

"They're selling like crazy," said William Lucy, a member of the national executive committee for Mandela's tour. "As I understand it, the $25 seats are closer and more in the center but all the tickets are good seats."

Money raised from the ticket sales will underwrite the cost of Mandela's three-day visit in Washington, which includes the Convention Center rent, his bill for his stay at the Madison Hotel and other expenses. Any profit will go to special funds set up for the anti-apartheid movement, organizers said.

"Chances are highly likely {the rally} will be broadcast in the District," said Christine Dolan, a press secretary for the national Mandela Welcoming Committee.

The Convention Center was chosen for the Mandela rally, over the much roomier, 55,000-seat Robert F. Kennedy Stadium, partly because of cost, said Aubrey McCutcheon, a volunteer on the local Mandela Welcoming Committee.

"It was more expensive to go to RFK stadium. That was a major consideration," McCutcheon said yesterday.

But Hill, the organizer in charge of ticket distribution, said other factors also were taken into account, such as a suitable public forum for Mandela "that would enable him to give a major address."

The local Mandela Welcoming Committee has held open meetings each Tuesday for the last several weeks to identify various sectors in the community where tickets should be distributed and sold, Hill said.

Representatives of those sectors "have a receipt for so many tickets and they are bringing the money to us," Hill said.

A limited number of complimentary tickets were given to Cedar Knoll and Oak Hill, the District's juvenile detention facilities, where the youths will be escorted to the Convention Center, Hill said.

About 300 marshals, residents picked from groups in the community that have contributed to the anti-apartheid movement, will help control the crowds and pass out programs.

"We feel very comfortable that we have reached a broad base of community groups, youth, civic groups, young, old, black and white," Hill said.

Staff writer Sharon Epperson contributed to this report.